Hello all I am starting to have an issue with my 350 SBC that literally just started today. It all started when I pulled over at a gas station after driving 30 minutes on the freeway to fill up. After starting the truck, I heard a loud knocking noise that went away after about 30 seconds or so. I noticed that the noise seemed to be coming from the driver side of the motor. I also noticed that the sound was loudest if I stood where in between the cab and the bed of my truck. Anyway, I pulled the driver side valve cover and checked the pushrods by twisting them with my index and thumb and some of them were loose. After tightening them up, I started the truck again and the problem went away. However, on my way home I stopped by a relative's house and after getting back in my truck and starting it, the sound came back. Except this time it lasted longer but eventually went away. I don't hear it while I'm driving only when I've shut the motor off and let it sit for probably 10 minutes and start it up again. Does anyone know what it might be?

S10 Racer

01-27-2013 07:25 PM

Quote:

Originally Posted by Chevyman87
(Post 1639465)

Hello all I am starting to have an issue with my 350 SBC that literally just started today. It all started when I pulled over at a gas station after driving 30 minutes on the freeway to fill up. After starting the truck, I heard a loud knocking noise that went away after about 30 seconds or so. I noticed that the noise seemed to be coming from the driver side of the motor. I also noticed that the sound was loudest if I stood where in between the cab and the bed of my truck. Anyway, I pulled the driver side valve cover and checked the pushrods by twisting them with my index and thumb and some of them were loose. After tightening them up, I started the truck again and the problem went away. However, on my way home I stopped by a relative's house and after getting back in my truck and starting it, the sound came back. Except this time it lasted longer but eventually went away. I don't hear it while I'm driving only when I've shut the motor off and let it sit for probably 10 minutes and start it up again. Does anyone know what it might be?

It sounds like you have a lifter bleeding down.

Chevyman87

01-27-2013 08:00 PM

Sorry but do you mind explaining what that means?

prumora1

01-27-2013 08:20 PM

reply

how many miles on engine, sounds like oil pump losing prime, or lifter bleeding off, try checking oil pressure on start up..

Chevyman87

01-28-2013 12:31 AM

I bought it from someone last year in May. When I bought the motor the seller told me it had 100,000 miles on it. The numbers in the cylinder head say its a 95 350 SBC.

cdminter59

01-28-2013 12:37 AM

Loud knocking when truck starts

Sounds like you need to adjust your valves. Like was said check your oil pressure to see if it is normal. Adjust the valves with the engine running. Loosen 1 rocker at a time until you hear it ticking. Tighten it until the ticking goes away. At this point turn the nut 1/2 turn more. Move on to the next valve until their all done. What type of oil are you using? Do you add ZDDP to your oil change every time? If you do not you are wearing the lobes off the camshaft.

Chevyman87

01-28-2013 12:51 AM

Won't that damage the engine to have the valve cover off while its running? Also, I checked the dipstick, I've been running the truck since December so it's not time for an oil change yet, but I noticed that the oil on the dipstick is normal color except for at the bottom of the dipstick there is some type of soot mixed in with the oil. I'm thinking maybe burnt oil but not too sure. The oil still has a thick consistency as well. Also, last month, I adjusted the valves because I noticed that some of the pushrods were really loose, but I did it with the motor off and I just manually turned the motor over.

cdminter59

01-28-2013 07:38 AM

Loud knocking when truck starts

Quote:

Originally Posted by Chevyman87
(Post 1639610)

Won't that damage the engine to have the valve cover off while its running? Also, I checked the dipstick, I've been running the truck since December so it's not time for an oil change yet, but I noticed that the oil on the dipstick is normal color except for at the bottom of the dipstick there is some type of soot mixed in with the oil. I'm thinking maybe burnt oil but not too sure. The oil still has a thick consistency as well. Also, last month, I adjusted the valves because I noticed that some of the pushrods were really loose, but I did it with the motor off and I just manually turned the motor over.

Won't that damage the engine to have the valve cover off while its running? Also, I checked the dipstick, I've been running the truck since December so it's not time for an oil change yet, but I noticed that the oil on the dipstick is normal color except for at the bottom of the dipstick there is some type of soot mixed in with the oil. I'm thinking maybe burnt oil but not too sure. The oil still has a thick consistency as well. Also, last month, I adjusted the valves because I noticed that some of the pushrods were really loose, but I did it with the motor off and I just manually turned the motor over.

No, it will not cause any damage but will cause a mess that will have to be cleaned up. A good way to avoid making a mess is to get a old set of valve covers from a junk yard. Then you can cut the top out of the valve covers and use them when adjusting valves. That soot you see, are you sure it is not metal powder from the cam and lifters. This would be the reason why they are loose all the time. Here is an article on premature camshaft wear and the reasons why. How to Keep Your Flat Tappet Engine from Getting Worn Down - Corvette Fever Magazine

cobalt327

01-28-2013 08:37 AM

Quote:

Originally Posted by Chevyman87
(Post 1639465)

Hello all I am starting to have an issue with my 350 SBC that literally just started today. It all started when I pulled over at a gas station after driving 30 minutes on the freeway to fill up. After starting the truck, I heard a loud knocking noise that went away after about 30 seconds or so. I noticed that the noise seemed to be coming from the driver side of the motor. I also noticed that the sound was loudest if I stood where in between the cab and the bed of my truck. Anyway, I pulled the driver side valve cover and checked the pushrods by twisting them with my index and thumb and some of them were loose. After tightening them up, I started the truck again and the problem went away. However, on my way home I stopped by a relative's house and after getting back in my truck and starting it, the sound came back. Except this time it lasted longer but eventually went away. I don't hear it while I'm driving only when I've shut the motor off and let it sit for probably 10 minutes and start it up again. Does anyone know what it might be?

Adjust the valves correctly. The push rods normally will be loose after the engine's been turned off for a while, so that's not how to tell if they need adjusting. Plus the lifter has to be on the lowest part of the cam (the base circle) when you adjust them.

You need to check the flexplate to be sure it's not cracked. Hearing the noise where you're hearing it (between the cab and bed) would mean it might be coming from aft of the engine. You'll need to get under it (block the wheels, always be safe!). With the engine Off, and using GOOD lighting, visually check it over, especially at the bolts holding it to the crank. Then use a short piece of 2X4 or a rubber mallet to rap on it to see if you get an odd sound- if it's cracked you should be able to hear it. You can also use a long screwdriver or pry bar to tweak the flexplate to see if it's cracked. Don't beat/bend it out of shape, but it can take some amount of force to get the crack to show itself.

One other thing that's often mistaken for a valve or engine knock is an exhaust leak at the header or manifold. I've heard leaks like that you'd SWEAR was a metal-on-metal noise, but it wasn't.;)

Chevyman87

01-29-2013 01:10 PM

I adjusted the valves and still nothing now it makes a weird noise when driving the truck. I pulled the spark plugs and they seemed fine. A buddy of mine helped me and we pulled the valve cover back off and turned the motor and no stuck valves. I have a recording of the noise but since I'm on my iPhone I don't think I can post I am stumped. I haven't experienced any loss of power just a shake while idling and a weird noise while driving.

cobalt327

01-29-2013 03:05 PM

Quote:

Originally Posted by Chevyman87
(Post 1640147)

I adjusted the valves and still nothing now it makes a weird noise when driving the truck. I pulled the spark plugs and they seemed fine. A buddy of mine helped me and we pulled the valve cover back off and turned the motor and no stuck valves. I have a recording of the noise but since I'm on my iPhone I don't think I can post I am stumped. I haven't experienced any loss of power just a shake while idling and a weird noise while driving.

Could be a vacuum leak. Is the sound you're hearing a whistle or sucking sound like air rushing through a small opening?

Check the vacuum lines- there's one that runs to the transmission (to the modulator valve). You can remove the vacuum lines from the vacuum ports of the carb and intake manifold one at a time and plug off the vacuum port w/your finger to see if the idle smooths out. If not, reattach it and go to the next vacuum port and do the same thing. Keep on until you either find one that makes the idle better, or you run out of vacuum ports.